This invention relates to a new process of preparing imides substituted on the nitrogen from amic acids as well as a process for the preparation of these imides from a di-acid or its anhydride and from a primary amine.
There is extensive literature on this subject. In accordance with Searle (C.A. 42 7340c (1948)), numerous authors have effected the preparation of N-imides, in particular N-aryl maleimides, by the cyclicizing dehydration of a maleamic acid in the presence of acetic anhydride as dehydrating agent and of alkaline acetate as catalyst, in accordance with the equation: ##STR2##
This reaction is, as a matter of fact, used as second step in the preparation of the above N-arylimides from an aniline and maleic anhydride. These two reagents give the intermediate, maleamic acid in a first step in accordance with the reaction: ##STR3##
This reaction takes place readily, the yields being generally greater than 90%.
This method, which was intended to be a general method, has encountered numerous failures, since the ease of reaction depends largely on the substituents borne by the phenyl ring. While, for example, yields of 70 to 90% are obtained when the substituents are chlorine atoms, they become average or even low when the phenyl ring is substituted by nitro, methyl, methoxy or ethoxy, or hydroxy groups or when the imide is substituted on the nitrogen by a lower alkyl radical.
To this there must be added the lack of reliability of the method, as illustrated by the great differences in yield indicated by various authors for one and the same product.
Finally, this method comprises two steps and provides impure, poorly crystalline products, the impurities not being capable of being eliminated by recrystallization. Such a process therefore absolutely does not constitute an industrial process.
Among the processes proposed in order to solve these problems, the one described in French Patent No. 1 418 336 constitutes an advance. This process consists in effecting the dehydration in solvent medium of an alpha or beta amic acid and particularly maleamic acid, no longer by means of a dehydrating agent but rather in the presence of 0.01 to 20% by weight, referred to the acid, of a catalyst formed of a strong acid such as sulfuric acid, or chlorosulfonic or organosulfonic acid, in particular paratoluene sulfonic acid, phosphoric acid and organophosphonic acid, particularly benzene phosphonic acid. This process leads to average crude yields of more than 50% while not exceeding 90%, but has several serious drawbacks. First of all, the crude products are very impure (from 5 to 20% impurities), and they must then be purified by chromatography on a column of alumina, which cannot be extrapolated industrially. Furthermore, the reaction times although shortened are still long. The example of the N-2-chlorophenylmaleamic acid of the patent cited, for which times of an hour are given, should not deceive one since its cyclization is particularly easy whatever the process contemplated. It should furthermore be pointed out that even in the case of this product, this reaction time may vary from one hour to nine and a half hours depending on the catalyst. As a matter of fact, when such a process is applied to a maleamic acid obtained from an aniline which is polymethylated on the ring, such as mesidine, in the presence of an effective catalyst such as paratoluene sulfonic acid, the time required is ten hours, which clearly shows the limitations of the process.
It has also been proposed (see West German Unexamined Application for Patent No. 2 100 800) to catalyze the dehydration of a succinamic or maleamic acid which has been obtained by condensation of succinic or maleic acid on an aromatic amine, the catalyst being produced by a strongly basic compound of the hydroxide or alcoholate type, of ammonium, alkaline or alkaline earth metal or tertiary amine, preferably triethyl amine.
In accordance with said patent, the process employs periods of time not in excess of four hours, and gives yields of the order of 80 to 90%, but has the serious defect of producing an impure product. Furthermore, its illustration is limited since, based on substituents in 3 or 3,5 position on the phenyl ring, and gives a single example of a monomethyl or monomethyloxyl product on the ring.
The application of this process to 2,4,6-trimethylphenylmaleamic acid has led to a very impure crude product (very highly colored), its melting point showing a difference of 10.degree. C. from the value for the pure product. Finally, this patent describes essentially xylene as solvent, which favors the reaction velocity, and gives an example in which when this solvent is replaced by toluene, again under reflux and therefore at lower temperature, the reaction cannot take place. Toluene is not included in the list of suitable solvents. This clearly shows the limitation of this process.